High temperature epoxy adhesive formulations

ABSTRACT

The invention is an epoxy resin system useful as an adhesive for high temperature applications. The system is a combination of a diglycidyl ether a bisphenol epoxy resin(s), at amounts of 30-70% by weight, with 1-10% by weight of an epoxy novolac resin 10-30% by weight of a polyurethane toughener which preferably has the terminal isocyanate functional group blocked, 1-8% by weight of a hardener, 0.1-% by weight of a cure accelerator, preferably a latent urea cure accelerator, provides the desired balance of mechanical strength and elastic modulus. The epoxy novolac resin is characterized by at least one of the following features: (i) having a viscosity at 25 degrees C. of less than 3000 mPa-s according to ASTM D-445, (ii) an average number of epoxide groups per molecule of more than 2 but less than 3.7, and (iii) a molecular weight of less than 750 g/mol.

FIELD OF THE INVENTION

This invention relates generally to epoxy formulations useful in bondingvarious surfaces that are anticipated to go through thermal stresses,including particularly use in bonding parts in transportation vehicles.

INTRODUCTION

In the manufacture of transportation vehicles, including especially carsand trucks, different materials are used in the design to reduce the thebody weight. Light weight materials like aluminum or carbon fiberreinforced composites are used in addition to steel. In addition,adhesives are more commonly used rather than just welding of parts.Since the vehicle body is heated during manufacture (e.g in the e-coatoven for corrosioin protection where temperatures may be in the range of160 to 210 C) and may also be subjected to large temperature rangevariations in use, the different thermal expansion rates of thedifferent materials can cause significant mechanical stresses in thejoints between the materials

Today's structural toughened epoxy based body shop adhesives are notentirely adequate to resist the thermal stresses caused by the differentcoeffeicients of linear thermal expansion (CLTE) of the parts made bydistinct materials. They are missing higher strength values at highertemperatures and cohesive failure modes at elevated temperatures.Adhesives that have a desired lower elastic modulus do not have theneeded high mechanical strength at high temperatures. Attempts toincrease the mechanical strength at high temperature often have an anundesired effect on the elastic modulus. Thus, a need exists for astructural epoxy adhesive formulation that achieves both a relativelylow elastic modulus (e.g. 600-1200 MPa) while retaining mechanicalstrength at elevated temperatures.

SUMMARY OF INVENTION

The present inventors have discovered a formulation which provides thedesired balance of mechanical strength at high temperatures and desiredelastic modulus.

Specifically, the inventors have found an epoxy resin system that is thecombination of a diglycidyl ether a bisphenol epoxy resin(s), at amountsof 30-70% by weight, with 1-10% by weight of an epoxy novolac resin10-30% by weight of a polyurethane toughener which preferably has theterminal isocyanate functional group blocked, 1-8% by weight of ahardener, 0.1-% by weight of a cure accelerator, preferably a latenturea cure accelerator, provides the desired balance of mechanicalstrength and elastic modulus. The epoxy novolac resin is characterizedby at least one of the following features: (i) having a viscosity at 25degrees C. of less than 3000 mPa-s according to ASTM D-445, (ii) anaverage number of epoxide groups per molecule of more than 2 but lessthan 3.7, and (iii) a molecular weight of less than 750 g/mol.

The invention is thus the above epoxy resin system and the curedreaction product of the above composition. According to one preferredembodiment the system comprises a single component of the aboveingredients where the polyurethane toughener is blocked so that it isstable with the epoxy and wherein the cure accelerator is latent andcure is initiated by heating, preferably to a temperature in the rangeof above 150° C. and preferably between 160 and 210° C. According to asecond embodiment the invention is also a system comprising a firstreaction component which is an epoxy resin component including thediglycidyl ether of bisphenol A and the epoxy novolac resin and a secondreaction component comprising the hardener and the cure accelerator.

DETAILED DESCRIPTION

The diglycidyl ether a bisphenol epoxy resins useful in this inventionmay be in liquid or solid form. A blend of such resins can be used.Preferably this component is a diglycidyl ether of bisphenol A,bisphenol F, bisphenol K, or bisphenol M. Most preferably it is adiglycidyl ether of bisphenol A.

If a blend is used it may be a mixture of a diglycidyl ether of at leastone polyhydric phenol, preferably bisphenol-A or bisphenol-F, having anepoxy equivalent weight of from 170 to 299, especially from 170 to 225,and at least one second diglycidyl ether of a polyhydric phenol, againpreferably bisphenol-A or bisphenol-F, this one having an epoxyequivalent weight of at least 300, preferably from 310 to 600. Theproportions of the two types of resins are preferably such that themixture of the two resins has an average epoxy equivalent weight of from225 to 400. The mixture optionally may also contain up to 20%,preferably up to 10%, of one or more other epoxy resin.

Suitable diglycidyl ethers include diglycidyl ethers of bisphenol Aresins such as are sold by Olin Corporation under the designationsD.E.R. 330, D.E.R. 331, D.E.R. 332, D.E.R. 383, D.E.R. 661 and D.E.R.662 resins. An example mixture of such resins include mixture of solidand liquid resins, such as D.E.R. 671 and D.E.R. 331 from Olin Corp. inamounts of 40 weight percent solid resin and 60 weight percent liquidresin and.

The amount of diglycidyl ethers of bisphenol A resins used is at least30 weight percent, and more preferably at least 40 weight percent but nomore than 70 weight percent, preferably no more than 60 weight percentbased on total weight of components of the system. Preferably, thediglycidyl ethers of bisphenol A (DGEBA) resins are a mix of solid andliquid epoxy resins with the solid DGEBA comprising at least 1 weightpercent, more preferably at least 2 weight percent but preferably notmore than 10 weight percent, more preferably not more than six weightpercent based on total weight of components of the system and the liquidDGEBA comprising at least 30 weight percent, and more preferably atleast 40 weight percent but no more than 60 weight percent based ontotal weight of components of the system.

The epoxy novolac resins useful in this invention are characterized by arelatively low viscosity and/or low molecular weight. Preferred epoxynovolac resins have the following structure

wherein n is the average number of repeat units and n is less than 1.7.Alternatively or in addition, preferred expoxy novolac resins haveviscosity at 25 degrees C. of less than 3000 mPa-s, preferably less than2500 mPa-s, and most preferably no more than 2000 mPa-s according toASTM D-445. Alternatively or in addition preferred epoxy novolac resinshave molecular weiths of less than 700 g/mol, preferably less than 650g/mol, more preferably still less than 600 grams/mol, still morepreferably less than 550 g/mol and most prefereably less than 500 g/molbut having a molecular weight of at least 250 grams/mol, preferably atleast 300 grams/mol. Examples of suitable epoxy novolac resins includeD.E.N. 354, D.E.N. 431 and D.E.N. 438 all from Olin Corporation.

The system of this invention comprises a polyurethane toughener.Preferred tougheners are chain extended by using a polyphenol,preferably ODBA (o,o′-diallylbisphenol A), using a combination of PTMEG(polytetramethylene glycol) and PBD (polybutadienediol) and are cappedby using a mono-phenol. Example F is devoid of any PBD and uses apolyphenol as capping group. Preferably, particularly for a singlecomponent system activated by heat, the toughener has blocked functionalgroups to prevent premature reaction. The blocking is preferably doneusing a monophenol. The polyurethane toughner is preferably apolymerized polytetramethylene ether glycol (PTMEG) in which the tipshave been modified with isocyanate and phenol compounds to allow them toreact with epoxy resin when curing conditions are obtained. In thesingle component heat-curable adhesives, the isocyanate prepolymer needsto be protected by a capping group that dissociates from thePU-toughener when exposed to heat (curing conditions). Preferably, thetoughener comprises residuals of diols have soft units (e.g. from PTMEG)and rubbery units (e.g. from polybutadienol) connected by residuals ofisocyantates. The polyurethane toughner may also be chain extended usinga polyphenol such a o,o′diallkylbisphonal a (OBDA). The capping groupsare preferably monophenolic groups such as cardanol (from cashew nutshell oil “CNSL”), or allyl phenol. A preferred polyurethane tougheneris the reaction product of PTMEG (10-95% by weight, preferably 20-90% byweight, more preferably 45-85% by weight) with a polybutadiene-diol(2-55% by weight, preferably 5-40% by weight, more preferably 10-30% byweight), a diioscyanate such as 1,6 hexamethylenediisocyanate (2-40% byweight, preferably 5-30% by weight, more prefably 10-20% by weight) andCNSL (0.1-50% by weight, preferably, 0.1-20% by weight, most preferably0.1-15% by weight) all based on weight of the toughener. Examples ofsuitable blocked polyurethane tougheners are those taught inWO/2017/044402 and U.S. Pat. No. 8,404,787 example 2, each of which isincorporated herein by reference.

The amount of toughener used is preferably at least 10 weight percent,more preferably at least 15 weight percent but preverably no more than30 weight percent, more preferably no more than 25 weight percent basedon total weight of the epoxy system.

The hardeners useful in this invention may by any known as useful in theart dicyandiamide, imidazoles, amines, amides, polyhydric phenols, andpolyanhydrides but are preferably amine or amide based hardeners. Thesehardeners are preferably latent such that they do not cause hardeningunder ambient conditions but rather must be heated or exposed to morethan normal lighting radiation exposure to cause hardening. Suitablehardeners include dicyandiamides such as Amicure™ CG 1200 from AirProducts or Dyhard 100 SF from Alzchem. The amount of hardener ispreferably at least one weight percent, more preferably at least 2weight percent, but preferably no more than 8 weight percent, morepreferably no more than 6 weight percent based on total weight of thesystem.

The curing accelerators useful in this invention are aromatic urea basedcure accelerators. These may be formed by reacting methylene diphenyldiisocyanate (MDI) with dimethylamines. Commercially available examplesof such aromatic uread based cure accelerators include Omnicure™ 52M and405M from Emerald. Phenyl-di-methyl based ureas are preferred. Theamount of cure accelerator is preferably at least 0.1, more preferablyat least 0.3 weight percent but preferably no more than 3, morepreferably no more than 1 weight percent based on total weight of thesystem.

In addition to the above, the composition also includes a flexibilizer.Suitable flexibilizers include carboxy terminated acrylonitrilebutadiene (CTBN) epoxy adduct or polyamine epoxy adducts. Examples ofsuch products include Strutkol 3604 from S&S Schill & Seilacher or acombination of a D.E.R. such as D.E.R. 330 from Olin Corporation with apolyether amine such as Jeffamine D-2000 from Huntsman Corporation. Theflexibilizer is preferably present in amounts of at least 5 weightpercent, more preferably at least 8 weight percent but no more than 20weight percent and more preferably no more than 15 weight percent basedon total weight of the components in the system.

Fillers may also be used when desired. Suitable fillers include calciumcarbonate, calcium oxide, talk wollastonite, glass beads, and the like.The amount of filler is preferably less than 30 weight percent, morepreferably less than 20 weight percent. When used, the amount of filleris preferably more than 5 weight percent.

The composition may also optionally include colorants, reactivediluents, adhesioin promoters and/or wetting agents. Examples ofsuitable colorants would be epoxy colorants from Huntsman CorporationAraldite DY series. Examples of reactive diluents include hexan, butyldiglycidyls and thereof. Adhesion promoters may be epoxy silanes suchas, for example, Sylquest A 187.

The single component system may be formulated by combining all thefluids and pastes, and solids in a mixer such as a planetary mixer. Fora two component system, the epoxy resin component including thediglycidyl ether of bisphenol A and the epoxy novolac resin may becombined and mixed and the second component including the hardener andcure accelerator may be combined and mixed. Then when it is desired touse the adhesive the two components are combined in sitiu.

The system is then heated to a temperature of at least 150° C.,preferably at least 160° C., but preferably no more than 210° C. tocure.

The adhesive system may be used according to any known process. Forexample, the mix may be filled in cartridges. The adhesive is commonlymanually applied for testing purposes out of cartridges through nozzlesby using a cartridge gun. If the viscosity of the adhesive formulationis that high, that manual application cannot readily performed at roomtemperature the cartridge can be heated prior to 40 to 60° C. for betterapplicability.

The inventive systems display a good balance of high mechanical strengthat elevated temperatures while having a relative low elastic modulus.Specifically, these systems show an elastic modulus of more than 600 butless than 1200 MPa when tested according to DIN EN ISO 527-1. Dumbbellspecimen 5a. They also a high Lap shear strength as evidenced by acohesive failure of 60 CF or more at 130 degrees C. and of 40 CF or moreat 140 degrees C.

Examples

The invention is further illustrated by the following non-limitingexamples.

TABLE 1 Raw material list for adhesive composition Supplier ChemistrySolid-liquid epoxy resin mix D.E.R Olin Corporation Solid/liquid DGEBAresin blend 671 to D.E.R. 331 = 40:60 BisA blocked PU toughener notcommercial WO 2005/007766 A1, preparation of toughener B, which has apolyfuncional capping group and lacks polybutadiene) Sec-amine blockedPU toughener not commercial U.S. Pat. No. 8,404,787 B2: example 2 D.E.N.438 Olin Corporation Epoxy-Novolac Struktol 3604 S&S Schill & SeilacherX8 CTBN-LER adduct: 60:40 Jeffamine-Epoxy adduct* See descriptionPolyamine-epoxy adduct Epoxy silane Momentive Silquest A 187 ColorantHuntsman Color pigment paste Polypox R18 Dow Reactive diluent: 1,6-Hexanediol-diglycidylether Dyhard 100 SF AlzChem micronizeddicyandiamide Curing accelerator: tris-2,4,6- not commercial U.S. Pat.No. 4,659,779: Polyvinylphenol tris(dimethylaminomethyl)phenol blockedManich base curing embedded into a polyvinylphenol accelerator.Accelerator II as polymer matrix described in the patent. Omicure U52MEmerald Urea curing accelerator Omicure U35M Emerald Urea curingaccelerator Omicure U405M Emerald Urea curing accelerator Omya BSH Omyacalciumcarbonate Nyglos 8 NYCO Wollastonite Huber 100C KaMin PerformanceCalcined Kaolin Minerals Talc 1N IMCD Deutschland, Hydrated magnesiumsilicate IMERYS Chaux Vive Lhoist Calciumoxide K25 3M Hallow glassspheres Fumed silica Evonik Like Aerosil 202 *DER 330 54.8 parts byweight and Jeffamin D-2000 45.2 parts by weight are added into a labreactor and heated up to 100° C. Mix the mixture for 60 min at 100° C.under vaccum. Then cool the mixture down to 60° C. and drum it.

TABLE 2 Raw material list for toughener A composition Amount (partsComp. Supplier Chemistry by weight) a PolyTHF BASFPolytetrahydrofuran-diol 2000 g/mol 57.58 2000 b PolyBD R45 CrayPolybutadien-diol 2800 g/mol 14.39 HTLO Valley c Desmodur H Bayer1,6-Hexamethylenediisocyanate 11.65 d Homid 127A HOSo,o′-diallylbisphenol A 5.74 e Cardolite NC Cardolite Cardanol 10.58 700f Dabco T12 N Air Dibutyltindialuerate 0.06 Products

General Toughener A (Preferred Toughener) Synthesis:

1. First reaction step: x wt % of component [a] and [b] are added into alab reactor and heated up to 120° C. Mix the mixture for 30 min at 120°C. under vacuum. Then cool the mixture down to 60° C. If the temp reach60° C. add x wt % of component [c] and let it mix for 2 min. Then add xwt % of component [d] and the mixture is allowed to react at 85° C.(bath temperature) for 45 min under nitrogen.2. Second reaction step: x wt % of component [e] is added and themixture is stirred for 60 min under nitrogen at 95° C. [bathtemperature].3. Third reaction step: x wt % of component [f] is added and the mixtureis stirred for 120 min under nitrogen. Finally the mixture will stirredfor 20 min under vacuum.Table 3 shows the various formulations made. These formulations weremade by following the general procedure:Combine all fluids, pastes and the colorant in the can and mix for fiveminutes at 50° C., then mix under vacuum for an additional 30 minutes.Scrape-down, add fumed silica, set temperature to 35° C., mix for threeminutes and then mix under vacuum for an additional 20 minutes. Add allother pigments, mix under vacuum for three minutes and then aftersraping down mix under vacuum for an additional fifteen minutes.

TABLE 3 Raw materials A B C D E F G H I J K L M N O Type ofexample-reference ref inv inv inv inv ref inv ref inv ref ref inv refref inv or inventive *Solid-liquid epoxy resin mix 29.92 26.82 24.7722.72 19.67 24.77 24.77 28.07 24.82 24.77 24.77 24.77 24.77 24.82 24.82D.E.N. 438 0 3 5 7 10 5 5 5 5 5 5 5 5 0 0 D.E.N. 439 0 0 0 0 0 0 0 0 0 00 0 0 5 0 D.E.N. 431 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 Struktol 3604 10 1010 10 10 10 10 0 0 10 10 10 10 10 10 INT Jeff-EP (table 4) 0 0 0 0 0 0 00 10 0 0 0 0 0 0 Toughener A 23 23 23 23 23 0 0 30 23 23 23 23 0 23 23Bis A blocked PU toughener 0 0 0 0 0 23 0 0 0 0 0 0 0 0 0 (RAM F)Sec-amine blocked PU 0 0 0 0 0 0 23 0 0 0 0 0 0 0 0 toughener (RAM DIPA)Sika D1 (US 2010/0273005A1) 0 0 0 0 0 0 0 0 0 0 0 0 23 0 0 (not chainextended, uses isophorone diisocyanate) Epoxy silan 0.8 0.8 0.8 0.8 0.80.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Polypox R18 2 2 2 2 2 2 2 2 2 22 2 2 2 2 colorant 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.530.53 0.53 0.53 0.53 0.53 DYHARD 100SF 3.15 3.25 3.3 3.35 3.4 3.3 3.3 3.03.25 3.3 3.3 3.3 3.3 3.25 3.25 Curing accelerator: tris-2,4,6- 0 0 0 0 00 0 0 0 0.7 0 0 0 0 0 tris(dimethylaminomethyl)phenol embedded into apolyvinyl- phenol polymer matrix Omicure U-52M (aromatic urea) 0.7 0.70.7 0.7 0.7 0.7 0.7 0.7 0.7 0 0 0 0.7 0.7 0.7 Omicure U-35M (aliphaticurea) 0 0 0 0 0 0 0 0 0 0 0.7 0 0 0 0 Omicure U-405M (aromatic 0 0 0 0 00 0 0 0 0 0 0.7 0 0 0 urea) Huber 100C 11 11 11 11 11 11 11 11 11 11 1111 11 11 11 Nyglos 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Talk 1N 0.2 0.2 0.20.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Chaux Vive 6 6 6 6 6 6 66 6 6 6 6 6 6 6 K25 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.71.7 1.7 Fumed silica 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7The formulations were tested according to the following procedures:

Rheology

-   -   Rotatory viscosity/yield stress: Bohlin CS-50 Rheometer, C/P 20,        up/down 0.1-20 s/l; evaluation according to Casson model        DSC: Mettler Toledo 25° C.-250° C., 10° C./min ramp up; TG        measured with the second run

Mechanical Testing

Used steel: hot dipped zinc coated steel: 420LAD+Z100 MB, thickness 1.2mm and electrolytically zinc coated steel HC 340LAD+ZE 50-50 thickness1.0 mm as supplied by Voest Alpine.

-   -   Lap shear strength following DIN EN 1465: 10×25 mm bonded area,        0.2 mm adhesive layer thickness. LS substrate=H420 Z100 LAD 1.2        mm//H340 ZE 50/50 1.0 mm combination supplied by VoestAlpine.        Test speed 10 mm/min.    -   Lap shear strength was tested at 23° C., 130° C. and 140° C.    -   Impact peel strength following ISO 11343: 20×30 mm bonded area,        0.3 mm adhesive layer thickness. IP substrate=H420 Z100 LAD 1.0        mm supplied by VoestAlpine. Test speed 2 m/s.    -   Modulus was tested following DIN EN ISO 527-1. Dumbell specimen        5a.        Table 4 shows the test results:

Formulation A B C D E F G H Type of Examples ref inv inv inv inv ref invref (reference or inventive) Lab number 1822.516 1822.515 1822.5171822.518 1822.519 1822.520 1822.521 1822.525 DSC, Tg [° C.] 81 78 79 9082 94 91 79 Initial rheological data Viscosity, Casson, 138 180 160 167129 225 58 170 45° C. [Pas] Yield stress, Casson, 877 768 759 694 800953 761 790 45° C. [Pa] Bulk adhesive data Elastic Modulus [MPa] 10161058 1107 1123 1026 1650 1065 943 Elongation [%] 11.5 11.1 10 9.3 8.17.3 11.5 11.1 Tensile Strength [MPa] 20 21 21 22 20 27 21 18 Lap shearstrength [MPa] steel HC 420 LAD + Z100MB/ 21.1 21.3 21.5 21.4 20.8 25.320.4 19 HC 340LAD + ZE 50-50 At 130° C. 4.5 5.6 6.1 6.4 7.4 8.0 7.0 5.3Failure mode at 130° C. 50CF 90CF 80CF 90CF 80CF 20CF 100CF 90CF At 140°C. 3.0 4.7 4.0 5.4 5.9 5.3 6 4.4 Failure mode at 140° C. 10CF 50CF40-50CF 50CF 60CF 10CF 100CF 10CF Impact peel strength [N/mm] HC420LAD + Z 100MB; 23° C. 30 29 28 26 23 33 32 29 Formulation I J K L M NO Type of Examples inv ref ref inv ref ref inv (reference or inventive)Lab number 1822.526 1822.527 1822.538 1822.539 1822.540 1822.5491822.555 DSC, Tg [° C.] 80 79 89 87 89 86 79 Initial rheological dataViscosity, Casson, 136 194 138 142 48 159 110 45° C. [Pas] Yield stress,Casson, 745 500 736 770 592 839 718 45° C. [Pa] Bulk adhesive dataElastic Modulus [MPa] 999 1431 1149 1057 1217 1210 1106 Elongation [%]10.9 7.2 8.7 9.5 7.3 10.3 10.4 Tensile Strength [MPa] 22 24 21 20 20 2120 Lap shear strength [MPa] steel HC 420 LAD + Z100MB/ 21.5 22 22.1 21.521.2 21.4 22.3 HC 340LAD + ZE 50-50 At 130° C. 6.8 6.8 5.9 6.2 5.4 6.48.1 Failure mode at 130° C. 85CF 90CF 70CF 80CF 40CF 20CF 70CF At 140°C. 4.3 4.0 4.7 4.3 4.5 4.6 5.7 Failure mode at 140° C. 70CF 10CF 10CF40-50CF 30-40CF 20CF 70CF Impact peel strength [N/mm] HC 420LAD + Z100MB; 23° C. 28 28 27 27 25 27 30

1. An adhesive epoxy resin system comprising a. 30-70% by weight of oneor more diglycidyl ether bisphenol A resins b. 1-10% by weight of anepoxy novolac resin wherein the epoxy novolac resin is characterized byat least one of the following features: (i) having a viscosity at 25degrees C. of less than 3000 mPa-s according to ASTM D-445, (ii) anaverage number of epoxide groups per molecule of more than 2 but lessthan 3.7, and (iii) a molecular weight of less than 750 g/mol c. 10-30%by weight of a polyurethane toughener, provided that if the system is asingle component system the polyurethane toughener is blocked with amonofunctional capping agent, d. 5-20% by weight of a flexibilizer e.1-8% by weight of a hardener, and f 0.1-% by weight of an aromatic ureabased cure accelerator, wherein the weight percents are based on totalweight of the system.
 2. The adhesive epoxy resin system of claim 1wherein the system is a single component system and the cure acceleratoris a latent cure accelerator.
 3. The adhesive epoxy resin system ofclaim 1 wherein the one or more diglycidyl ether bisphenol A resinscomprise a liquid expoxy resin component in an amount of 30-60% byweight of the system and a solid epoxy resin component in an amount of 1to 10% by weight of the system.
 4. The adhesive epoxy resin system ofclaim 1 further comprising a filler in an amount of 5 to 30% by weight.5. The adhesive epoxy resin system of claim 1 wherein the epoxy novolacresin has the formula

wherein n is the average number of repeat units and n is less than 1.7.6. The adhesive epoxy resin system of claim 1 wherein the polyurethanetoughener is formed from is the reaction product of polytetraalkyleneether glycol with a polyalkadiene-diol and a diioscyanate
 7. Theadhesive epoxy resin of claim 6 wherein the toughener is chain extendedby reaction with a polyphenol chain extender.
 8. The adhesive epoxyresin of claim 7 where in the chain extender is o,o′-diallylbisphenol A.9. The adhesive epoxy resin system of claim 1 wherein the flexibilizeris a carboxy terminated acrylonitrile butadiene epoxy adduct or apolyamine epoxy adduct.
 10. The adhesive epoxy resin system of claim 1wherein the hardener is a dicyandiamide.
 11. The adhesive epoxy resinsystem of claim 1 wherein the epoxy novolac resin has a viscosity at 25degrees C. of less than 3000 mPa-s according to ASTM D-445.
 12. Theadhesive epoxy resin system of claim 1 wherein the epoxy novolac resinhas an average number of epoxide groups per molecule of more than 2 butless than 3.7
 13. The adhesive epoxy resin system of claim 1 wherein theepoxy novolac resin has a molecular weight of less than 750 g/mol. 14.The adhesive epoxy resin system of claim 1 which is a two componentsystem wherein a first component comprises the one or more diglycidylether bisphenol A resins and the epoxy novolac resin and a secondcomponent comprises the polyurethane toughener, the hardener and thecure accelerator.